Piston construction



NOV. 11, 1941. o, Q BERRY 2,262,132

PISTON CONS TRUCTION Filed June 7, 1940 2 Sheets-Sheet l @i l 1r NOV.11, Q ;l BERRY PISTON CONS TRUCTION Filed June '7, 1940 2 Sheets-Sheet 2PISTON PIN PISTON PIN AXIS AXIS TG P 0F SKIRT CYLINDER Orro nete-.e5seen l I l @E Patented Nov. 1l, 1941 UNITED STATES PATENTV GFFICE2,262,132 rls'roN CONSTRUCTION tto Carter Berry, Ann Arbor, Mich.

Application June 7, 1940, Serial No. 339,232

12 Claims.'

This invention relates to pistons for internal combustion engines, andmore particularly to aluminum alloy pistons for use in automobileengines and the like.

The principal objects of this invention are the following:

To provide an aluminum alloy piston so constructed that the clearancebetween the cylinder and the sides of the piston skirt that receive. theside thrust not only varies with the temperature of the cylinder, as hasbeen customary in the past, but also is almost entirely independent o!thetemperature of the piston head.

To provide a piston that will not collapse, that is, become permanentlysmaller across the main diameter of the skirt, even under the mostsevere working conditions. I

To provide a piston in which the open end of the skirt is especiallywell prepared to keep the oil scraped down in the cylinder and thusimprove oil economy.

In a pistonmade according to my invention, the skirt is oval in shapeand includes a strong continuous portion or band in the upper part, andthis upper part expands when the head of the piston is heated becauseheat is transmitted to it from the head. The principal feature of myinvention is the provision of means to cause the upper part of the skirtto contract across its major diameter as the temperature of the headrises and the balancing of this contraction against the thermalexpansion caused by heat from the head, so that, with, the cylinder atany constant temperature, the major diameter of the top of the skirtwill stay constant in spite of variations in the temperature of thehead.

Another important feature of my invention is the provision of a pistonskirt that is oval in cross-section to permit the skirt to bear againstthe cylinder at both sides even when cold and also be able to expandwithout binding, but which is more nearly circular at thebottom than atthe top, so that there will not be any unnecessary looseness at thebottom of the skirt under the pin bosses.

These and other objects and features of the invention will be more fullyapparent from the following description and claims and the accom- Figure3 is a section taken on the line III--III of Fig. 2 looking in thedirection of the arrows;

Figure 4 is a horizontal section taken on the line IV--IV of Fig. 1, andlooking up in the direction of the arrows;

Figure 5 is a side elevation of another form of the invention;

Figure 6 is a horizontal section, similar to Fig. 4, of a third form ofthe invention;

Figure '7 is adiagram showing the relative shape of the piston and acylinder in which it nts, the diierences in shape being greatlyexaggexited in ,order that they may be readily seen; an

Figure 8 is a diagram illustrating a modiiication of one part of theinvention. i

The form of the invention shown in Figs. 1 to 4 and illustrateddiagrammatically in Figs. 7 and 8 consists of a piston having a head I0,a skirt II, and wrist pin bosses I2 formed of an aluminum or similaralloy and an expansion control band I3 `formed of steel or similarmetal. The head I0 of the piston is provided with the conventionalgrooves I4, I5 and IE for receiving the piston rings and is separatedfrom the skirt II at both sides of the piston by horizontal slots I1 andI8.

The horizontal slots II and I8 terminate some distance from each otherso that connecting portions or bridges of solid metal remain directlyabove the piston pin bosses I2 andrconnect the skirt II to the head I0.These connecting portions |9, the piston pin bosses I2 and the head I0are braced and reinforced by a deep vertical rib 2| extending from` onepiston pin boss I2 up inside and across the head I0 and down to theother piston pin boss. It may be noted from Fig. 2 that the rib 2| is sodeep that its lower edge is below the plane of the horizontal slots IIand I8. The exact depth of the rib 2| is not critical, but it isimportant that it be very deep, for reasons which will be explainedbelow.

The top 22 of the skirt II, immediately below the horizontal slots I'Iand I8, is much thicker than the remainder o`f the skirt and containsthe expansion control band. 'I'he steel expansion control band I3 ismanufactured from a length of steel rod having its ends Welded togetheras at 22 to form a continuous band, and

it is placed in its proper position relative to the' remainder of thepiston by having the aluminum portion of the piston cast around it. Thisis accomplished by placing the band I3 in the mold in which the pistonis to be cast, the band I3 being supported in the mold by four lugsprojecting out from the core portion of the mold.

These lugs leave four small recesses 23 where the band I3 is exposed,but these recesses 23 are onA the interior of the piston and do notmatter. Adhering of the aluminum to the steel band I3 during thecastingy ofthe aluminum is prevented by previously coating the band I3with graphite by the method described in U. S. Patent 1,431,936 to JohnThomson or by any other suitable method, such as by polishing the bandwith a cloth dusted with deocculated graphite.

One side of the piston skirt II is formed, during the casting operation,with a vertical slot 24 extending from the horizontal slot I'I towardbut terminating short of the open end of the skirt..

Although the slot 24 severs the upper portion of the skirt II, the bandI3 embedded in the top of the skirt is continuous and extends unbrokenacross the slot, as clearly shown in Figs. 1 and 4. In thisconstruction, the graphite with which the band I3 is coated facilitatessliding of the skirt on the band so that, when the skirt and band areheated and the sk irt expands relative to the band, the portions of theskirt at either side of the vertical slot 24 will slide toward eachother on the band I3 so that said band I3 and not the aluminum of theskirt controls the circumference of the skirt at the top. The weld 22'is preferably placed diametrically opposite from the vertical slot 24 sothat the band I3 is anchored against sliding at this point, thusreducing the sliding to a minimum.

'I'he skirt II is not exactly cylindrical, but the variation from a truecylindrical shape is so small as to be not apparent in Figs. 1 to 4,which are drawn to scale, and it is therefore shown greatly exaggeratedin Fig. 7. As shown in Fig. '7, the skirt is not circular in crosssection, but is ground to a slightly oval shape, and the amount ofdeviation from a true circle is greater at the top of the skirt than atthe bottom. The major diameter of the skirt should be substantially thesame at the top and the bottom, and the greater deviation from a truecircle at the top than at the bottom is caused by making the minordiameter of the skirt decrease from the bottom to the top of the skirt.The grinding of the piston skirt into oval shape, known as cam grindingis done in such a way that the major or larger diameter of the pistonskirt is at right angles to the axis of the piston pin, so that thesides of the piston which receive the side thrust caused by theangularity of the connecting rod will bear against the walls ofthe-cylinder, while the sides where the piston pin bosses are locatedwill be spaced away from the cylinden The surface of the piston skirtneed not be a complete smooth oval or ellipse, as shown in Fig. '7, butreliefs may be provided in the regions of the wrist-pin bosses, asillustrated diagrammatically in Fig. 8. If such reliefs are provided, itis possible to grind the remainder of the skirt circular and still havethe hoop stretching effect present. Such skirts, as well as any othersthat might function in the same general way, are included within theterm oval, as used in this specii'lcation and the accompanying claims.`

When the piston described above is placed in a cold cylinder and-isitself cold, there will be a slight clearance between the pistonskirtand the walls of the cylinder. Under these conditions, if therewere no oil in the cylinder, the piston would be free to move a minutedistance from side to side and would slap in operation. However, thewalls of the cylinder are normally covered with oil, and, when thecylinder is cold,

the oil is also cold and is comparatively thick. The thick oil prevents"slapping of the piston, and the relatively large clearance between thepiston and the cylinder allows the engine to be yturned over with` areasonable amount of force when it is being started cold.

If the temperature of both the piston andthe cylinder is raised to thenormal operating temperature of the cylinder without any part of thepiston being raised to a higher temperature, as is substantially thecase in an engine whose cooling system is provided with a thermostat andwhich is allowed to idle for some time, the piston will expand relativeto the cylinder, because the piston is formed chiefly of an aluminum orsimilar alloy having a coefficient of expansion roughly twice as greatas that of the iron forming the cylinder. The bottom of the skirtexpands and presses against the sides of the cylinder, and it might bindif it did not have the oval shape illustrated diagrammatically in Fig.'7. The oval shape permits the expansion of the bottom of the skirt totake place in a direction parallel to the piston pin axis. Thedifference in shape between the bottom of the skirt and the cylinderwhen the piston is cold is made great enough so that, at the maximumtemperature which the piston and the cylinder attain, the bottom of theskirt will just ll the cylinder with the minimum permissible workingclearance.

When the piston and cylinder are raised to a uniform workingtemperature, the top of the skirt will expand less than the bottom ofthe skirt because of the presence of the steel band I3, and, because ofthis, the\ p of the skirt will merely expand out to the si es of thecylinder when the piston is heated up to the operating temperature ofthe cylinder. Thus, if the entire j' piston is heated uniformly to thenormal operating temperature of the cylinder, the clearance both at thetop and the bottom of the skirt will be reduced to what is desirablewhen the oil is hot. At the bottom, the skirt will press resilientlyagainst the sides of the cylinder and scrape oi excess oil on thedownstroke, while at the top hotter than the cylinder walls, althoughnot quite.

as hot as the piston head. At first glance, it would" appear that theincrease in temperature of the top of the skirt relative to the cylinderwalls under these conditions would cause the top of the skirt to expandand iit more tightly in the cylinder. The remarkable thing about thispiston is that the top of the skirt does not t any tighter in thecylinder when the head of the piston is very hot than when the head ofthe piston is relatively cool. Thisremarkable result is attained by whatmay be called a hoopstretching 'action which, briefly, is as follows.

When the engine in which the piston is used is operating at fullthrottle, the entire upper surface of the head of the piston is exposedto very hot gases during each cycle of operations in the cylinder andabsorbs heat from these gases. A small part of this heat is absorbedfrom the head of the piston by oil splashed against the under surface,but this cooling eiect of the oil is relatively unimportant inautomobile engines. The major portion of the heat absorbed by the pistonhead fiows out to the side of the-head and down into the skirt and istransferred to the cylinder walls by the piston rings and the pistonskirt. A considerable amount of heat is also carried from the pistonhead to the piston skirt and to the wrist pin bosses I2 by the deepVertical rib 2| on the under side of the head. Because of this iiow inheat, the hottest point on the piston is at the center of the head, andthe coolest point is at the bottom of the skirt, which is always atsubstantially the temperature of the cylinder walls. The deep verticalrib 2| on the bottom of the head attains a temperature which liesbetween the temperature of the head and the temperature ofthe skirt, thecentral portion of this rib becoming almost as hot as the hottestportion of the head and the outer portion of the rib 2| becomingslightly hotter than the adjacent portion of the skirt.

The deep vertical rib 2| and the top of the head I form a T-beam joiningthe two piston pin bosses I2. When this beam I02I becomes hotter thanthe piston skirt, as it does whenever the head I0 becomes hotter thanthe piston skirt, it forces the two wrist pin bosses I2 apart. When thewrist pin bosses I2 are forced apart, they carry with them the adjacentportion of the skirt Il. If the skirt II had heated up and expanded asmuch as the head I0 and rib 2|, the entire piston would have increasedin size without materially changing its shape, and the only result wouldbe to make it fit more tightly in the cylinder. vThis does not happenfor two reasons: First, the top of the skirt does not get as hot as thehead I0 and rib 2|; and, second, its effective coefficient of expansionis much lower because of the presence of the steel band I3. Since thetop of the skirt does not expand as much as the head. the forcing apartof the wrist pin bosses I2 causes the skirt to be distorted from itsoriginal oval shape (shown in full lines in Figure 7) towards aperfectly round shape (shown in dotted lines in Figure 7), thustendingto pull vaway from the cylinder walls the two sides of the pistonskirts which are closest to it. If the piston is correctly designed,this tendency will be exactly balanced by the thermal expansion of thetop of the piston skirt and the diameter of the piston skirt at rightangle to the axis of the piston pin will not change. Thus, the clearancebetween the top of the piston skirt and the cylinder wall in a directionat right angles to the piston pin axis will remain constant for a givencylinder wall temperature even though the temperature of the head of thepiston may vary greatly.

It will be noted that the clearance in a direction parallel to thepiston pin axis will change considerably with the temperature of thehead of the piston, but this is not detrimental for all of the sidethrust on a piston comes in a direction at right angles to the pistonpin axis. The one important thing is to maintain the correct clearanceat right angles to the piston pin axis, and this result is obtained bythis piston.

It may be noted that the maintaining of a constant major diameter at thetop of the piston skirt depends upon the hoop-stretching effectbalancing out or counteracting the effect of the thermal expansion ofthe upper portion ofthe using a heavier expansion control band I3;secondl by using an expansion control band I3 having a lower coefficientof thermal expansion; and third, by lengthening the horizontal slots Il'and I8 and thus decreasing the amount of heat that iiows to the skirtfrom the head. The hoop-stretching effect can be increased, first. bymaking the vertical rib 2| deeper or thicker; second, by lengthening thehorizontal slots I'I and I8; and third, by making the upper portion ofthe skirt thinner so that it will flex more easily. The hoop-stretchingeffect will be decreased by cross ribs extending on the underside of thepiston head transverse to the piston pin axis and joined at their endsto the ring ange, and it may be diilicult or impossible to secure enoughhoop-stretching effect if such ribs are used. It more bracing of thehead than is given by one rib is desired, two or more deep ribs parallelto each other and to the piston pin axis may be used, and they willcause the hoop-stretching effect in substantially the same way as asingle deep rib.

Besides being an important factor in causing the hoop-stretching effect,the deep vertical rib 2| on the under side of the head I0 performs thefollowing important functions. It helps to carry heat away from thecentral portion of the piston head I0 out to the ring grooves I4, I5 andI6 and down to the skirt, thus reducing the temperature of the pistonhead. It strengthens the head I0 and enables it to better withstand thepressure of the explosion in the cylinder. It distributes both theexplosion forces and the inertia forces acting on the piston evenlyalong the piston pin bosses I2. All of these functions improve theoperation of the piston and lengthen its life and thus materially helpto make a better piston.

There are as yet no empirical rules that will enable one to design apiston having an exact balance between the thermal expansion of theskirt and the hoop-stretching effect. It is skirt. In order to securethe desired balance,

there are several factors affecting it which can be necessary to proceedby the cut and try method until the desired result is reached. Thepiston illustrated in Figures 1 to 4, for example, is designed tooperate in a 31's inch cylinder and has the following characteristics:The aluminum alloy is the one commonly employed for pistons andcontaining 'I per cent of copper and 5 per cent of silicon. 'Ihe band isof S. A. E. 1020 steel and is made of /s inch rod. 'I'he clearance inthe cylinder when the cylinder and piston are cold is .0015 inch acrossthe major diameter both at the top and the bottom, and the cam grindingis such that there is .014 inch relief at the top of the skirt in theneighborhood of the pin bosses and .009 inch at the bottom.

A second form of the invention is shown in Figure 5. In this form of theinvention, every portion of the piston is similar to the correspondingportion of the piston shown in Figures 1 to 4 and described above withthe exception of the arrangement of slots in the piston skirt. In thisform of the invention, there are horizontal slots similar to thehorizontal slots I1 and I8 in the first form of the invention, betweenthe skirt II' and the head I0. One side of the piston skirt II' isslotted vertically, just as in the piston shown in Figures 1 to 4, butinstead of there being a single central vertical slot 24, there are twovertical slots 24', one extending down from each end of the horizontalslot I1'. Thus, this piston4 is a special form of U-slot piston ascompared to the piston shown in Figures 1 to 4, which is a special formof T-slot piston. The piston shown in Figure contains a deep verticalrib and a steel expansion control band, similar to the vertical rib 2land the band I3 in the first form of piston, and it operates insubstantially the same manner.

Another form of the invention, which is illustrated in Figure 6, isexactly like the forms shown in Figures l to 5 except that there are novertical slots 24 or 2l'. In this form of the invention, the skirt II"contains the steel expansion control band I3" and is separated from thehead I0" by horizontal slots at either side of the piston. The head I0"is provided with a deep central vertical rib 2| which causes thehoopstretching action described above'. In 'this form of the invention,if the same materials and proportions are used as in. the piston shownin Figures 1 to 5, there will be a greater circumferential expansion ofthe upper portion of the piston skirt as its temperature increases. However, the desired balance between the circumferential expansion of thevupper portion of the skirt and the hoop-stretching effect" can besecured by changing the factors enumerated above, such as by making theband I3" m'uch` stronger in comparison to the upper portion of thepiston skirt or by making it of a steel or other metal having a muchlower coefficient of thermal expansion. In manufacturing this form ofpiston, there is no need of coating the band I3" with graphite, as therewill be no movement between it and the portion of the skirt II" in whichit is embedded. I

- Besides the advantages which have already been mentioned, it has beenfound that pistons of the type described above and shown in the drawingsact very efficiently as cross-heads or guides for the upper end of theconnecting rod. The bottom of the piston skirt expands and lits thecylinder very closely as soon as the cylinder reaches its normaloperating temperature, and

vit therefore positions the bottom of the piston very accurately. Sinceit fits around the side of the cylinder, and since it is unslotted, itcannot be greatly deformed by any lateral or other pressure applied toit. Although the top of the piston skirt is slotted in some of the formsof `the piston, the slots are bridged by the expansion control bands,and the upper portion of the skirt cannot be compressedcircumferentially. Since the upper portion of the piston skirt is rmlybraced across its minor diameter by the deep rib 2|, and is stiened bythe expansion control band and the relatively thick mass of metal inwhich the band is embedded, it is very stiff against any deformationthat might be caused by the side thrusts of the connecting rod or otherexternal forces. -Thus, there is a minimum of deflection of either thetop or the bottom of the piston as it is subjected to the various forcesthat act upon it during operation. This absence of side motion orofcooking in the cylinder is a factor of great importance in obtaininghigh oil economy and in maintaining low blow-by over a long period ofservice.

Pistons have been made with ribs under the bosses. Pistons have alsobeen made with exof the ribs in those pistons caused some hoopstretchingaction, not one of them was intended pansion control bands inthe pistonskirt, as in stretching action, and in none of them was a deep rib usedin combination with an expansion control band in the piston skirt. Whilesome to be or was capable of making the hoopstretching actioncounterbalance the part of the thermal expansion of the skirt thatresulted from the hot head. No one realized that it was possible tosecure such a balance and thus make the piston t well irrespective ofthe temperature of the head. That is my discovery, and a pistonproportioned so that it will function in that way, is my mai invention.

While I have illustrated and described several of the preferredembodiments of the invention, it is to be understood that this has beendone by way of illustration and that the invention is not confined tothese. The pistons have been described as being made of aluminum alloy,but the invention includes pistons of other light alloys, such as thoseof magnesium, if other satisfactory light alloys should be produced.Also. the pistons have been described as being for use in cast-ironcylinders, but they may obviously be used in cylinders of othermaterial, such as nitralloy, and the term cast-iron as used herein is tobe understood as including those other cylinder materials. My invention,therefore, includes every vconstruction falling within the terms of anyone or more of the following claims, with the terms of the claims giventhe lof the skirt, and including means responsive to said transferredheat for hoop-stretching the top of said skirt at said junction so as toflex inwardly said contacting portions, and to thus balance out thethermal expansion of the top end of the skirt due to the hightemperature of the piston head, said skirt being of oval cross sectionand having a vertical slot only in its top end directly below the planeof its junction with the head, andhaving a horizontal control band offerrous material bridging said vertical slot which has a considerablylower rate of thermal expansion than the material of said skirt, so thatas the piston is heatedfrom a uniform low to a uniformly highertemperature the major diameter of the end of said skirt will expand at arate less than that of the material of said skirt, but more than that ofsaid ferrous control band.

2. In a metallic hollow piston of the type including a head and a. skirtjoined to the head in the regions of the pin bosses and separated fromthe head in regions at right angles to said pin bosses, providing theskirt with relatively flexible cylinder contacting portions at saidseparated regions adapted to be hoop-stretched, interior heat transfermeans connected to the piston head top and connected to the skirtinsubstantially the plane of the pin bosses, and expansible by the. hightemperatured heat of the piston head topto pull in at right anglesthereto said flexible cylinder contacting portions for hoop-stretchingthe top of said skirt at substantially the plane of its junction withsaid head,

so as to balance out the thermal expansion due the material of saidskirt, so that as the piston is heated from a uniform low to a uniformlyhigher temperature the major diameter of said skirt through the flexibleportions will expand at a rate less than that of the material of saidskirt, but more than that of said ferrous band.

3. An aluminum piston for use in a cast iron cylinder of an internalcombustion engine comprising a head with a ring flange, a skirt carryingWrist-pin bosses and having the upper portion formed substantially ovalin shape with the major diameter perpendicular to the axis of said wristpin bosses, said skirt being connected to said ring flange in theregions of said wrist pin bosses and separated from said ring flangebetween said regions to form relatively flexible cylinder-bearingportions in said skirt that are adapted to be hoop-stretched, a controlband of metal having a lower coelcient of thermal expansion than saidskirt secured to the upper portion of said skirt,

and a rib extending across the under side of said.

head parallel to the axis of said wrist pin bosses,

Vthe bottom of said rib being in the region of the plane of said controlband, said parts being so constructed and arranged that as the piston isheated from a uniform low to a uniformly higher temperature the maiordiameter of the top end of said skirt will expand enough more than castiron of the cylinder and less than the aluminum of the piston to providereduced clearance between the cylinder and piston, and when said pistonhead is heated above the temperature of the other parts of said pistonunder service conditions, the maior diameter of the head end of saidskirt will not be changed by the change in said piston head temperature.

4. An aluminum piston for use in a cast iron cylinder of an internalcombustion engine comprising a headwith a ring flange, a skirt carryingwrist pin bosses and having the upper portion formed substantially ovalin shape with the major diameter perpendicular to the axis of said wristpin bosses, said skirt being connected-to the ring flange in the regionsof said wrist pin bosses and separated from Asaid ring flange betweensaid regions, said skirt having an upright slot extending down from thetop of one of said separated portions, and a ferrous band secured to theupper portion of the skirt and bridging said upright slot. said skirtbeing continuous on the other of said separated portions on the sideopposite to said upright slot, said parts being so constructed andarranged that when the temperature of the piston is raised from auniform low to a uniformly higher temperature, the major diameter at thetop end of said skirt is expanded at a rate less than that of aluminumof the piston and enough more than cast iron of the cylinder, to producereduced clearance between the piston and cylinder.

5. In a piston as defined in claim 1 further characterized by saidvertical slot being in one of said cylinder contacting portions of theskirt and stopping short of the skirt bottom.

6. In a piston as defined in claim 1 further characterized by saidvertical slot being in one of said cylinder contacting portions of theskirt and stopping short of the skirt bottom. said skirt being of anoval cross section which progressively changes from the top to thebottom thereof, so that the skirt at its bottom end 'has across-sectional shape more closely approaching a circle than at its topend.

7. In a piston as defined in claim 1 further characterized by saidVertical Slot being in one of said cylinder contacting portions of theskirt and stopping short of the skirt bottom, said skirt being of anoval cross section which progressively changes from the top to thebottom thereof, so that the skirt at its bottom end has across-sectional shape more closely approaching a circle than at its topend, said bottom and open end of the skirt being formed to have a snug tin the cylinder whereby it is effective in scraping down oil on thedownstroke of the piston.

8. In a metallic hollow piston of the type including a head and a skirt,said skirt being joined to the head in the regions of the pin bosses andseparated from the head between said pin bosses to formcylinder-contacting portions adapted to be'vhoop-stretched, means on theinterior of the piston head for transferring the relatively hightemperatured heat of the top of the piston head through the interiorthereof to the plane of the junction of said head and skirt, andincluding means responsive to said transferred heat for hoop-stretchingthe top of said skirt at said junction so as to flex inwardly saidcylinder-contacting' portions and to thus balance out the thermalexpansion of the top end of the skirt due to the high temperature of thelpiston head.

9. In a metallic hollow piston the structure defined in claim 8 and inwhich the heat transferring and hoop-stretching means includes avertically depending projection from the top of the piston head andterminating substantially at the plane of the junction of the head andskirt.

10. An internal combustion engine piston comprising a head with a ringflange, a skirt carrying wrist-pin bosses and having its upper portionformed substantially oval in shape with the major diameter thereofperpendicular to the axis oi' said wrist-pin bosses, said skirt beingconnected to said ring flange inthe regions of said wrist-pin bosses andseparated from said ring flange between said regions to form cylinderbearing portions that are adapted to be hoopstretched, and a ribextending across the under side of said head parallel to theaxis of saidwristpin bosses, the bottom of said rib being in the region of the planewhere said head and skirt are joined together, said parts being soconstructed and arranged that when said head becomes hotter than saidskirt, the hoop-stretching action balances out the thermal expansion ofthe main diameter of the top end of said skirt that is due to said headbeing hotter than said skirt.

11. In a piston as defined in claim 8 characterized by the skirt beingof an oval cross section which progressively changes from the top to thebottom thereof so that the skirt at its bottom end has a cross-sectionalshape more closely approaching a circle than at its top end.

12. In an engine piston as defined in claim 10. further characterized bythe skirt being o! an oval cross section which progressively changesfrom the top to the bottom thereof, so that the skirt at its bottom endhas a cross-sectional shape more closely approaching a circle than atits top end and so that the bottom end can have a snug nt in a cylinderwhereby it is eiective in scraping down oil on the downstroke of thepiston. y OTIO CARTER BERRY.

